1. Field of the Invention
The invention relates to the field of microwave monolithic integrated circuits (MMICs) for wireless applications, such as cellular radio and personal communication systems. More particularly, it relates to MMICs which include passive circuit elements and active amplifier stages which are coupled together by microwave transmission line in order to achieve reduced propagation loss of microwave signals conveyed there-between.
2. Description of the Related Art
In an MMIC passive circuit elements such as resistors, inductors and capacitors are fabricated in monolithic form on a silicon substrate. Active amplifier stages are also fabricated in the monolithic structure, and are coupled to the passive elements by microwave transmission line. This has commonly taken the form of a microstrip line, which consists of a conducting strip on the upper surface of a silicon dioxide dielectric layer and a metal ground plane on the back surface. An MMIC structure of this kind is shown, for example, in FIG. 8-161 on page 105 of the text Electronics Engineers Handbook, McGraw Hill Book Co., 3d ed. 1989.
A major problem with such a structure is that the electric field component of a quasi-TEM electromagnetic wave must couple to the ground plane through the entire substrate, resulting in high propagation losses. This problem is avoided by providing a ground plane on the same surface of the silicon die as the conducting strip, resulting in a coplanar waveguide (CPW) structure. This type of waveguide is also shown in the aforesaid FIG. 8-161 of the cited textbook. The small spatial separation between the conducting strip and the ground plane effectively confines the electric field to the near-surface of the substrate, reducing the depth of penetration into the body of the substrate. The transmission loss, which is the inverse of the quality factor Q, is then dependent on the conductivity and consequently the doping level of the near-surface region of the substrate. By heavily doping to achieve a high conductivity the transmission loss can be reduced, increasing the quality factor Q. However, it is time-consuming and expensive to produce a heavily doped silicon substrate body.
The technical paper "Properties of Microstrip Line on Si--SiO.sub.2 System" by Hasegawa et al, IEEE Trans. On MTT, Vol. MIT-19, No. 11, Nov. 11, 1971, describes a microstrip transmission line having a layer of SiO.sub.2 on a moderately doped silicon substrate. Under some conditions, this achieves a "slow-wave" mode of propagation, having reduced transmission loss. However, this structure does not lend itself to monolithic fabrication with other circuit elements and still has higher transmission loss than is desirable. The technical paper "Propagation Characteristics of MIS Transmission Lines with Inhomogeneous Doping Profile" by Wu et al, IEEE Trans. on MTT, Vol. 38, No. 12, December 1990, describes a coplanar waveguide having an inhomogeneously doped silicon substrate, and achieves relatively low transmission loss. However, the inhomogeneous doping must extend through the entire thickness of the bulk silicon substrate, which is very difficult to achieve in practice.